Break up the gelantine leaves and place in a small bowl and vocer with just enough water to cover. Soak for 1½ hours.
Place the blackberries, water, sugar and lemon juice into a pan over a medium heat. Bring to the boil, then reduce the heat to simmer until the fruit has broken down completely to a sauce.
Place a sieve lined with two layers of muslin over a bowl and strain the blackberry mixture into the bowl. (Don't press the pulp, as this will make for a cloudy jelly.) Once all the liquid has drained through discard the fruit pulp.
Place the collected liquid into a clean pan and cook over a low heat, until just warm, then whisk in the gelatine and water. Make sure you dissolve any lumps.
Remove from the heat and add the sloe gin.
Pour the mixture into a 700ml/1 pint 3fl oz jelly mould and cover with a plate. Chill overnight in the fridge.
To serve, fill a large pan or bowl with hot water to just under the level of the jelly in its mould. Place the mould into the hot water and leave there for no more than 20 seconds.
Place a plate over the top of the mould, remove from the water and turn over. Lift the mould off the jelly carefully and serve. (If the jelly does not come loose, return to the hot water for another ten seconds, then try again.)